patriarch of Constantinople from 446 to 449, who opposed the heretical doctrine of the monophysites (q.v.). He presided at the Synod of Constantinople (448), which condemned the monk Eutyches (q.v.), proponent of an extreme form of Monophysitism. Pope St. Leo I the Great approved the synod's action in his famous Tome (449). Patriarch Dioscorus of Alexandria, on behalf of Eutyches, influenced the Eastern Roman emperor Theodosius II to summon the Robber Synod of Ephesus (Ephesus, councils of) (Aug. 8, 449), which exonerated Eutyches and deposed Flavian, whose opponents beat him to death three days later.

The Council of Chalcedon (451) vindicated him, praising him as a saintly martyr.